This invention relates to an exhaust-gas recirculation system for an internal-combustion engine, and more specifically to such a system improved so that the amount of exhaust-gas recirculation can be automatically increased during engine acceleration.
A variety of methods have been practiced to reduce the concentrations of oxides of nitrogen (hereinafter called "NOx" for simplicity) in the exhaust gases from internal-combustion engines. Of those attempts, the exhaust-gas recirculation ("EGR") by which the exhaust gas is partly returned to the combustion chamber has been known to be most effective in decreasing the NOx emissions.
A typical EGR system so far proposed includes a vacuum-operated EGR valve mounted midway in an EGR pipe providing communication between the intake and exhaust systems of the engine, in such a manner that the EGR valve is communicated with the portion of the intake system near the throttle valve therein through an exhaust-pressure transducer valve and via a vacuum passage. The term "exhaust-pressure transducer valve" as used herein means an arrangement by which a vacuum passage communicating the intake system of the engine with an EGR valve is opened midway to the atmosphere by an atmospheric-releasing orifice and the orifice, in turn, is opened or closed by a valve body fast on a diaphragm responsive to the exhaust pressure on the exhaust side of an EGR pipe in which the EGR valve is mounted, so that the negative pressure or vacuum being introduced into the EGR valve can be controlled.
The control of the vacuum introduction into the EGR valve by use of the exhaust pressure is resorted to because it permits the exhaust gas recirculation according to the rate of air flow into the engine. The exhaust pressure being approximately proportional to the rate of air intake, it follows that, if the exhaust pressure is used to control the entry of vacuum into the EGR valve, then the rate of exhaust gas recirculation can be increased as the air intake increases. With such an EGR system, however, it has been found that when the engine is accelerated by opening the throttle valve in the intake system the NOx emissions are large in the early stage of acceleration. This is attributable to the fact that in the early stage of acceleration with the throttle open the rate of exhaust gas recirculation is inadequate because it fails to catch up with the throttle opening accordingly, and hence the increased NOx emissions during the early period of acceleration.